Literature on "Thin Slice" ratings indicates that a number of personality characteristics and behaviors can be accurately predicted by ratings of very short segments (<5?min) of behavior. This study examined the utility of Thin Slice ratings of young children with autism spectrum disorder for predicting developmental skills and language gains over time. A total of 22 preschool-aged children with autism spectrum disorder participated in a battery of developmental assessments and a video-taped therapist-child interaction at Time 1. They then participated in follow-up testing of language skills and a second therapist-child interaction 6?months later (Time 2). Groups of approximately 25 naïve undergraduate students provided impression ratings ("Thin Slice ratings") about each child's skills and behaviors during 2-min segments taken from the therapist-child interaction videos at each time point. Thin Slice ratings at Time 1 were highly correlated with child scores on several developmental assessments at Time 1. In addition, Thin Slice ratings at Time 1 predicted gain in parent-reported expressive vocabulary over the course of 6?months, over and above the predictive utility of Time 1 vocabulary size. These findings provide preliminary evidence for the concurrent and predictive validity of Thin Slice ratings in young children with autism spectrum disorder.